High speed integrated circuits use copper to form the metal lines that connect the various electronic devices that comprise the circuit. Copper lines are formed using a damascene process that is illustrated in FIGS. 4-5(a) and FIGS. 4-5(b). As shown in FIGS. 4-5(a), a dielectric layer 310 is formed over a semiconductor 300. The semiconductor will contain electronic devices such as transistors that are omitted from the Figure for clarity. In a typical simply damascene process, a trench 315 is first formed in the dielectric layer 310. A barrier layer 320 is then formed over the surface of the dielectric layer and in the trench. Typical materials used to form the barrier layer include titanium nitride and other similar materials. Following the formation of the barrier layer 320, a copper a layer 330 is formed. The copper layer is typically formed using a plating process and in addition to filling the trench 315, forms excess copper over the entire semiconductor surface. The excess copper is removed using chemical mechanical polishing (herein after CMP) resulting in the structure shown in FIGS. 45(b). The remaining copper line 315 is then used to interconnect various electronic devices that are formed in the semiconductor.
During the CMP process a wafer is placed facedown on a rotating wafer holder. A slurry material is placed on a rotating polishing pad and surface of the wafer is brought in contact with the polishing pad thereby removing the targeted material from the surface of the wafer. A critical component of any CMP process is the endpoint detection. In the case of copper if the polishing process is stopped too soon then copper will remain on the surface rendering the circuit inoperable. If the polishing process continues beyond the optimum endpoint then dishing of the copper surface or erosion of the dielectric will occur leading to the presence of defects in the completed circuit or high sheet resistance of the metal. It is therefore crucial that an accurate measure exist to detect the desired endpoint of any CMP process. For many CMP processes the endpoint occurs during the transition from a first material to a second material. This is illustrated in FIG. 4(b) where the transition from copper 330 to the underlying barrier layer 320 will signal the removal of all the excess copper from the surface of the wafer.
In one common CMP tool configuration, an optical endpoint detection system is used whereby light of one or more wavelengths is reflected off the polish surface of the wafer during the polish process and then collected by a detector. The change in the reflected light is detected as a signal and is based on the change in the reflective properties of the polished surface as it polishes (i.e. the transition from a metal reflective surface to a barrier layer surface). The signal is compared to a standard or baseline determined for some sample of material processes in this fashion (i.e., experiments are run on a set of wafers to determine the average endpoint characteristics of the “typical” wafer endpoint signal to collected signal of the next wafer to process.) The problem with this approach lies in the comparison of the current endpoint signal to the baseline signal. During the CMP process, variation from a number of sources causes the collected signal to be quite different from the expected signal, resulting in early, late, or an altogether missed endpoint, any of which can have a marked impact on the device structure, electrical performance and long term reliability. In addition the reference point for the endpoint signal detection is set within the set of data collected from the wafer as it is processed. Therefore, on a wafer-to-wafer basis, the reference point for the endpoint signal is not a constant and introduces additional variability into the process.
There is therefore a need for an endpoint detection method that reduces the variability of existing methods. The instant invention addresses this need.